Image forming apparatuses, such as copying machines, facsimile machines, printers, or multifunction apparatuses having two or more functions of copying, printing, scanning, and faxing, generally use an electrophotographic process for image forming and includes a fixing device.
In the fixing device, a heating member heats toner in a toner image formed on a recording sheet to fix the toner image onto the recording sheet. In order to suppress toner from being adhered to the surface of the heating member, a lubricating device is provided that applies lubricant such as oil to the surface of the heating member. For example, as illustrated in Japanese Patent Application Publication Nos. H09-54512-A, H06-79189, S58-182673, S61-46965, S62-44783, H10-307503, 2003-5562, 2004-37556, and 2008-102412, oil may be applied to the surface of the heating member via a felt, which is made in close contact with the heating member. More specifically, in the example case of JP-H09-54512-A, which is illustrated in FIG. 1, a felt 23 is reeled from a felt subsidiary roller 20 to a felt main roller 21 such that the contact surface of the felt 23 that is made in contact with the heating member is kept clean. In order to reel the felt 23, a drive motor 22 is provided, which rotates the felt main roller 21 at a constant angular velocity. However, as the felt 23 is reeled in, the outer diameter of the felt main roller 21 including the felt 23, which is the diameter of a rotational axis of the felt main roller 21, increases, resulting in the decrease in the circumferential velocity of the felt 23.
In view of the above-described problem, JP-H09-54512-A proposes to detect a rotational velocity of the felt subsidiary roller 20 and control the rotation velocity of the felt main roller 21 based on the detected rotational velocity, using a rotation velocity detector 24. The rotation velocity detector 24 includes an encoder disc 26 provided at one end of a rotational shaft 25 of the felt subsidiary roller 20, and an encoder sensor 27 to monitor a rotational state of the encoder disc 26.
However, the conveyance velocity of the felt conveyance mechanism is generally very slow, for example, 1 cm per hour. Accordingly, it requires a relatively long time to obtain the angular velocity of the felt subsidiary roller 20.
Further, the speed or time required for the felt 23 to be reeled may differ depending on paper, toner being used, setting temperature for the fixing device, and so forth. Furthermore, the purpose for reeling the felt may differ between when the image forming apparatus is performing a printing operation and when the image forming apparatus is not performing a printing operation. More specifically, in a printing operation, the felt is moved at a predetermined velocity to clean the heat roller 14, and to coat the heat roller 14 with oil. However, in a non-printing operation, if the felt is pressed against the heat roller without movement, the felt may be deformed due to the heat of the heat roller. Accordingly, the felt should be moved at a velocity that is slower than the velocity during printing. Thus, a plurality of velocities should be detected and determined to reflect both cases of printing operation and non-printing operation.
Referring to FIG. 2, it is assumed that a pulse generation cycle, i.e., the rotational cycle of the felt subsidiary roller 20 is Tp, a rotational velocity of the drive motor 22 in a printing operation is μp, and a rotational velocity of the drive motor 22 in a non-printing operation is μd. As previously described, the rotational velocity μd of the drive motor 22 at the non-printing operation is generally slower than the rotational velocity μd of the drive motor 22 at the printing operation. Even if the rotational velocities of the drive motor 22 are the same, the actual felt feeding speeds may be different depending on reeled condition of the felt 23.
Further, FIG. 2 represents a case in which a diameter of the felt main roller 21 in state B is larger than the diameter of the felt main roller 21 in state A. However, the pulse generation cycles Tp in both states may be the same, and in this case, according to the known lubricating device of FIG. 1, the same rotational velocity may be set in both cases even if the diameters of the felt main roller 21 actually differ between the two cases. As a result, the rotational velocity for reeling the felt 23 may become too fast or too slow.